Port GrGLCaps over to use SkTHash.

Port GrGLCaps over to use SkTHash.

I've written some new hashtable interfaces that should be easier to use,
and I've been trying to roll them out bit by bit, hopefully replacing
SkTDynamicHash, SkTMultiMap, SkTHashCache, etc.

This turns the cache in GrGLCaps::readPixelsSupported() into an SkTHashMap,
mapping the format key to a bool.  Functionally, it's the same.

BUG=skia:

Review URL: https://codereview.chromium.org/948473002